1. Field of the Invention
The present invention relates to the making of paper and paper boards and in particular, to a method and apparatus for adjusting fibre treatment apparatus to produce paper having desired qualities.
2. Description of the Prior Art
In the machine manufacture of paper and cellulose products, the paper making materials, known as "stock" usually consist of a water suspension of wood or vegetable fibres which have been separated by chemical or mechanical means. These fibres are combined with additional ingredients such as dye stuffs, alum, clay and the like, to impart the desired characteristics to the finished paper product.
Preparation of the fibre stock is a most critical processing phase, which greatly determines the quality and physical characteristics of the finished product. The properties of wood and natural fibres vary, even in the same plant species, due to differences in factors affecting growth. Other variations are introduced by the chemical and mechanical processes which reduce timber logs to individual cellulose fibres. To improve uniformity, the final preparation of the fibres prior to forming into a sheet on the paper machine is by treating the slurry of fibres in machines known as beaters and refiners. These machines typically have rotating parts faced with metal or abrasives, and corresponding fixed parts similarly faced with metal or abrasives.
During operation the rotating parts are maintained at a controlled very small spacing from the adjacent corresponding fixed metal or abrasive faced parts. The fibres are crushed by being pumped in a slurry through the spacing. The fibres which are originally relatively smooth and tubular in shape are smashed, and their exteriors develop fine fibrils which interlock or "felt" together when the slurry is formed into a sheet, contributing greatly to the strength of the finished product.
By practical experience, an optimum degree of refining is determined for any specific product. Under-refining may produce a low strength product; over-refining may affect opacity, texture and other qualities, in addition to wasting considerable energy.
Several beaters and refiners operating in parallel are required to prepare stock for a single paper machine. For practical reasons such as routine maintenance, the internal mechanical condition of the refiners will vary. For example reconditioning of the refiners is performed in sequence by shutting down a single unit while others in the parallel system continue to operate. Thus, a set of refiners feeding a single paper machine may consist of some new or freshly reconditioned units, while others are worn and ready to be reconditioned. The effect of such unequal refiners on wood fibres will vary greatly, the reconditioned machines requiring less energy input to produce the desired refining result than is required of a worn machine. The new or reconditioned machines may also tend to cut a greater proportion of fibres than will a worn machine.
To achieve the desired degree of fibre treatment, the common practice in paper mills is to control the beaters and refiners according to a predetermined energy input per ton of fibre stock, in terms of Horsepower Tons per Day. The fibre slurry is passed through the beaters or refiners until a desired horsepower per hour has been expended per ton of fibre in the system. But as noted above, a uniform energy input will not necessarily produce a uniformly treated fibre stock, as the internal condition of the machines may vary.
Hitherto, the degree of refining achieved for a particular mixture of fibres has not been able to be confirmed until the fibre has been formed into a dried sheet, samples of which are periodically subjected to a series of physical tests to check quality. To provide for uninterrupted feeding of the continuously running paper machine, yet allow for product and formulation changes, the beaters and refiners discharge into large holding tanks which are fed in sequence to the paper machine. Thus, a delay of up to several hours may occur before a batch of refined stock is formed into sheets so that the result of refining can be confirmed. If the physical tests indicate that under or over-refining has occurred, any correction made at the beaters or refiners to alter the energy input per ton of stock will not show an effect on the finished paper qulity for a long period, during which a non-standard product will continue to be made. The cost in wasted energy and possible price reduction of a lower grade product may be considerable.
It is an object of the present invention to provide a method of making paper and an apparatus for making paper in accordance with the method which will overcome, or substantially ameliorate, the abovementioned disadvantages.